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A model organism is a species that is extensively studied to understand particular biological and psychological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. This is possible because fundamental biological principles such as metabolic, regulatory, and developmental pathways, and the genes that code for them, are conserved through evolution.

Often, model organisms are chosen on the basis that they are amenable to experimental manipulation. This usually will include characteristics such as; short life-cycle, techniques for genetic manipulation (inbred strains, stem cell lines, and transfection systems), and non-specialist living requirements. Sometimes, the genome arrangement facilitates the sequencing of the model organism's genome, for example, by being very compact or having a low proportion of junk DNA.

When researchers look for an organism to use in their studies, they look for several traits. Common among these are size, generation time, accessibility, manipulation, genetics, conservation of mechanisms, and potential economic benefit. As comparative molecular biology has become more common, some researchers have sought model organisms that represent assorted lineages of life.

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Caenorhabditis elegans, a nematode, usually called C. elegans[1] - an excellent model for understanding the genetic control of development and physiology. C. elegans was the first multicellular organism whose genome was completely sequenced

Oryzias latipes, Medaka (the Japanese ricefish) is an important model in developmental biology, and has the advantage of being much sturdier than the traditional Zebrafish.

Rat (Rattus norvegicus) - particularly useful as a toxicology model; also particularly useful as a neurological model and source of primary cell cultures, owing to the larger size of organs and suborganellar structures relative to the mouse. (Molecular evolution, Genomics)'

Zebrafish (Brachydanio rerio), a freshwater fish, has a nearly transparent body which provides unique visual access to the animal's internal anatomy throughout its life. Zebrafish are used to study development, toxicology and toxicopathology[3], specific gene function and roles of signaling pathways.

↑Spitsbergen J.M. and Kent M.L. (2003). The state of the art of the zebrafish model for toxicology and toxicologic pathology research--advantages and current limitations. Toxicol Pathol.31 (Supplement), 62-87. PubMed Abstract Link => PMID 12597434.